Pulsed-continuous etching

ABSTRACT

In a system and method of etching a sample disposed in an etching chamber, a plurality of separately stored charges of an etching gas is discharged, one at a time, into a sample etching chamber. The discharge of each charge of etching gas occurs such that a momentary overlap exists in the end discharge of one charge of etching gas with the beginning discharge of another charge of etching gas, whereupon the desired flow of etching gas into the etching chamber is maintained. During discharge of one charge of etching gas, a previously discharged charge of etching gas is recharged. The process of discharging a plurality of separately stored charges of an etching gas, one at a time, and recharging at least one previously discharged charges of etching gas during the discharge of at least one charge of etching gas continues until the sample is etched to a desired extent.

Vapor etching of semiconductor materials and/or substrates is accomplished using gases such as xenon difluoride. Specifically, in xenon difluoride etching, the xenon difluoride gas reacts with solid materials such as silicon and molybdenum such that the materials are converted to a gas phase and removed. This removal of these materials is known as etching.

Adding non-etching gases have been described by Kirt Reed Williams, “Micromachined Hot-Filament Vacuum Devices,” Ph.D. Dissertation, UC Berkeley, May 1997, p. 396¹, U.S. Pat. No. 6,409,876, and U.S. Pat. No. 6,290,864, to the xenon difluoride can offer improvements to the etch process. The advantages of using non-etchant gases to xenon difluoride etching gas are noted in U.S. Pat. No. 6,290,864 which include improved selectivity, which is the ratio of etching of the material to be etched versus those materials that are intended to remain and uniformity. Increases in both of these parameters ultimately lead to improved yield. ¹ Kirt Reed Williams, “Micromachined Hot-Filament Vacuum Devices,” Ph.D. Dissertation, UB Berkeley, May 1997, p. 396.

A common approach to xenon difluoride etching is through the pulsed method of etching.² In this mode, xenon difluoride is sublimated from a solid to a gas in an intermediate chamber, referred to as an expansion chamber, which can then be mixed with other gases. The gas(es) in the expansion chamber can then flow into an etching chamber to etch the sample, referred to as the etching step. The main chamber is then emptied through a vacuum pump and this cycle, including the etching step, is referred to as an etching cycle. These cycles are repeated as necessary to achieve the desired amount of etching. ² Chu, P. B.; J. T. Chen; R. Yeh; G. Lin; J. C. P. Huang; B. A. Warneke; K. S. J. Pister “Controlled PulseEtching with Xenon Difluoride”; 1997 International Conference on Solid State Sensors and Actuators—TRANSDUCERS '97, Chicago, USA, June 16-19, p. 665-668

Alternatively, xenon difluoride etching can be accomplished using a continuous method such as that described in McQuarrie et al., U.S. Pat. No. 6,409,876 where single reservoir is connected to a flow controller to provide a constant flow of xenon difluoride gas to the sample to be etched. In addition, a means of mixing an additional, inert, gas to the etch gas between the outlet side of the flow controller and the inlet of the chamber is described.

Adding an additional gas, typically an inert or minimally reacting gas, such as nitrogen, to the etching process, must be accomplished keeping in mind the sublimation pressure of xenon difluoride. Often, the partial pressure of the additional, non-etching gas is higher than the sublimation pressure, which is the pressure below which that xenon difluoride is a gas and above which is a solid, of the xenon difluoride. At 25 C, the sublimation pressure of xenon difluoride is approximately 4 torr. It is not uncommon during pulsed etching to mix in high pressures of other gases, such as nitrogen, into the expansion chamber, after the expansion chamber has been filled with a few torr of xenon difluoride, to high pressures such as 30 torr. However, in a continuous process, such as that described in U.S. Pat. No. 6,409,876, the pressures of the additional gas mixed into the xenon difluoride would have to be less than the pressure of the supplied xenon difluoride gas. The reason for this limitation is that additional gas pressures higher than the xenon difluoride pressure between the outlet of the flow controller and inlet to the chamber would cause the xenon difluoride to stop flowing through the controller.

We herein describe a process sequence to allow the continuous flowing of xenon difluoride gas with mixture of high pressures of additional gases. To maintain long duration, continuous etches, it uses multiple expansion chambers, which allows one expansion chamber to be used for etching while the other is being prepared.

The gases can be any inert gas such as helium, nitrogen, or argon. Mixtures of inert gases are also possible. Note that the term inert is used to refer to any gas that minimally reacts with the etching chemistry and is also referred to as a non-etching gas.

In addition, other vapor phased etching gases, such as bromine trifluoride, could be used in addition to or in place of xenon difluoride.

DETAILED DESCRIPTION OF THE INVENTION

Shown in FIG. 1, a vapor etching gas source 120, which is usually a cylinder of gas, such as xenon difluoride, is connected to a shutoff valve 1118. Shutoff valves 112 and 114 are connected to expansion chambers 106 and 108 which are used as an intermediate chamber to regulate the quantity of etching gas in each cycle. The expansion chambers 106 and 108 can be optionally independently evacuated through shutoff valves 111 and 115. The expansion chambers 106 and 108 also have pressure sensors 105 and 107 which are typically capacitance diaphragm gauges. In addition, the expansion chambers have additional connections to shutoff valves 116 and 117 to allow mixing gases such as nitrogen to be mixed with the xenon difluoride in the expansion chambers. In series with shutoff valves 116 and 117 can also be a needle valve and additional shutoff valves to provide additional control of the flow of the incoming mixing gases.

The expansion chambers 106 and 108 are connected to the main chamber 123 via a flow path that includes shutoff valves 109 and 110 which then split into two paths, one through a flow controller 101 with additional shutoff valves 100 and 102 or another which bypasses the flow controller 101 via shutoff valve 104. The flow controller is one that is designed for controlling flow with low pressure drops such as those designed for SDS, or Safe Delivery Systems, like those provided by Celerity.

Xenon difluoride gas can also be introduced into the main chamber 123 without flowing through the expansion chambers 106 or 108 by flowing directly through shutoff valve 113.

The main chamber can be vented, or filled with an inert gas to raise the pressure to atmosphere for opening, via shutoff valve 103. This shutoff valve could alternatively be located on the flow path to the chamber on the other side of shutoff valve 104.

The main chamber pressure is monitored using a pressure sensor 122 which is preferably a capacitance diaphragm gauge. The pressure in the main chamber 123 is controlled using an automatic pressure controller 124 which adjusts the conductance between the main chamber 123 and the vacuum pump 126. Such pressure controllers are available from MKS Instruments. The vacuum pump is preferably a dry vacuum pump. In addition, the connection between the chamber 123 and the vacuum pump 126 can be fully isolated using vacuum valve 125.

Not shown in the figures is that the system is controlled using either a computer or other similar controller, such as a programmable logic controller. Manual operation is possible but is difficult.

Other modifications to the aforementioned system design are envisioned such as those described in U.S. Pat. No. 6,887,337 (assigned to XACTIX) including, but not limited to, variable volume expansion chambers, more than two expansion chambers, and multiple gas sources. The addition of multiple gas sources is shown in FIG. 2 where additional gas source 121 and valve 119 have been added. Additional gas sources could be added in a similar fashion.

In addition, the use of other noble gas fluorides, such as krypton difluoride or halogen fluorides, such as bromine trifluoride, are also considered for etching. In addition, combinations of these gases are also considered.

A typical etching sequence is to load the sample into the main chamber 123. The main chamber 123 is then evacuated through opening vacuum valve 125 which connects the vacuum pump 126 to the main chamber 123. Typically, the main chamber is pumped down to 0.3 Torr. The main chamber 123 may be further purged of atmosphere by first closing vacuum valve 125, opening shutoff valves 103 and 104, and flowing the venting gas, which is typically nitrogen, into the chamber to approximately 400 Torr (anywhere from 1 Torr to 600 Torr would be useful, though). These pumps and purges are repeated typically three or more times to minimize moisture and undesired atmospheric gases in the chamber 123. Most critically, moisture can react with xenon difluoride and other etching gases to form hydrofluoric acid which will attack non-silicon materials.

The etching sequence then proceeds generally as described in FIG. 3. Expansion chamber one (106) is evacuated through shutoff valve 111, typically to around 0.3 torr as monitored by 105, and is then filled to the desired pressure of etching gas as monitored by 105 by opening and then closing shutoff valves 118 and 112. Expansion chamber one can then be further filled with the additional mixing gas to a specific pressure as monitored by 105 by opening and then closing shutoff valve 116. The second expansion chamber 108 can then be similarly prepared for use through the control of shutoff valves 115, 118, 114, and 117 using 107 to monitor the pressure.

The preparation of the second expansion chamber 108 can be executed while the first expansion chamber 106 is being used for etching. To use the first expansion chamber 106 for etching, the flow controller 101 is set to a given flow rate, typically in the range of a few standard cubic centimeters (sccm) of flow. The pressure controller 124 is also set to reach a specified pressure, typically around one torr. Etching commences by opening shutoff valves 109, 100, 102, and 125. During this time, the flow of the gas mixture will be controlled to the setpoint and the pressure in the chamber will also rise to its setpoint. As the etch progresses, the pressure in the expansion chamber 106 will fall and the flow controller 101 will need to continue to open its control valve. Once the valve is nearing approximately 90% of fully open, there is sufficiently likelihood that the flow rate through the flow controller 101 will begin to drop below the setpoint.

After the indication that the flow is about to drop below the setpoint, shutoff valve 109 is then closed and shutoff valve 110 is then opened so that the etching gas mixture is coming from expansion chamber two 108. Immediately following this change between expansion chambers, expansion chamber one 106 is then evacuated and refilled so that it is ready for use when expansion chamber two 108 can no longer support sufficient etching gas mixture flow. This cycle of alternating between expansion chambers 106 and 108 continues until the end of the desired etching time has been reached.

It should be noted that although the valve position in the flow controller 101 is one way to measure the capacity of an expansion chamber to support a given flow, other means including examining the pressure in the expansion chamber via sensors 105 or 107 is also possible. In the case of examining the expansion chamber pressure, determinations from look-up tables, previous results, or analytical models can be used to decide at what pressure to switch between expansion chambers during an etch.

It should also be noted that during the switch between expansion chambers that the pressure on the inlet side of the flow controller will rapidly increase. To counteract this sudden pressure jump, it may be necessary to make a preemptive adjustment to the valve position in the flow controller 101 when switching between expansion chambers. As described in U.S. Pat. No. 6,887,337, variable volume expansion chambers can be used which can be collapsed in a continuous fashion to maintain a constant pressure at the inlet of the flow controller 101. However, in this case, it would be necessary to incorporate the percent that the expansion chamber has been collapsed to decide when to switch between expansion chambers. Specifically, when one expansion chamber is nearing fully collapsed, the other expansion chamber should be used. It should be noted that the pressure at the inlet of the flow controller can be controlled by the speed at which the expansion chamber is collapsed during the etch. 

1-19. (canceled)
 20. A method of etching a sample via an etching system having an etching gas source, a main chamber where etching of a sample occurs, a first expansion chamber, a second expansion chamber, and means for connecting each expansion chamber to the main chamber and the source of etching gas, the method comprising: (a) controlling the means for connecting to charge the first expansion chamber with a suitable amount of etching gas from the etching gas source to sustain a constant flow of etching gas to the main chamber, and once charged is isolated therefrom; (b) controlling the means for connecting to connect the charged first expansion chamber to the main chamber, whereupon the charge of etching gas in the first expansion chamber flows to the main chamber such that the pressure of etching gas inside the first expansion chamber decreases; (c) while etching gas in the first expansion chamber is flowing to the main chamber, controlling the means for connecting such that the second expansion chamber is charged with a suitable amount of etching gas from the etching gas source to sustain a constant flow of etching gas to the main chamber, and once charged is isolated therefrom; (d) following step (c), controlling the means for connecting to isolate the first expansion chamber from the main chamber and to connect the second expansion chamber to the main chamber before a pressure of the etching gas in the first expansion chamber decreases below a level sufficient to sustain a constant flow of etching gas to the main chamber; (e) following step (d), while etching gas in the second expansion chamber is flowing to the main chamber, controlling the means for connecting such that the first expansion chamber is charged with a suitable amount of etching gas from the etching gas source to sustain a constant flow of etching gas to the main chamber, and once charged is isolated therefrom; and (f) following step (e), controlling the means for connecting to isolate the second expansion chamber from the main chamber and to connect the first expansion chamber to the main chamber before the pressure of the etching gas in the second expansion chamber decreases below a level sufficient to sustain a constant flow of etching gas to the main chamber.
 21. The method of claim 20, further including repeating steps (c)-(f).
 22. The method of claim 20, wherein, in steps (d) and (f), the means for connecting is controlled to simultaneously connect the first and second expansion chambers to the main chamber.
 23. The method of claim 22, wherein the first and second expansion chambers are simultaneously connected to the main chamber momentarily.
 24. The method of claim 22, wherein: the etching system includes means for preventing etching gas from flowing from the first expansion chamber to the second expansion chamber, and vice versa, when the first and second expansion chambers are simultaneously connected to the main chamber; in step (d), the means for preventing is operative for preventing the flow etching gas from the second expansion chamber into the first expansion chamber when the first and second expansion chambers are simultaneously connected to the main chamber; and in step (f), the means for preventing is operative for preventing the flow etching gas from the first expansion chamber into the second expansion chamber when the first and second expansion chambers are simultaneously connected to the main chamber.
 25. The method of claim 20, wherein, at least one of step (a), step (c) and step (e) includes controlling the means for connecting to charge the corresponding expansion chamber with an inert gas from an inert gas source coupled to the means for connecting.
 26. The method of claim 25, wherein the inert gas is nitrogen, helium, argon, xenon or some combination thereof.
 27. A vapor etching system comprising: a source of etching gas; a main chamber where etching of a sample occurs; a first expansion chamber; a second expansion chamber; means for connecting each expansion chamber to the main chamber and the source of etching gas; and a controller operative for performing the steps of: (a) controlling the means for connecting to connect the first expansion chamber to the source of etching gas to be charged with a suitable amount of etching gas to sustain a constant flow of etching gas to the main chamber when connected thereto, and once charged to be isolated therefrom; (b) controlling the means for connecting to connect the charged first expansion chamber to the main chamber, whereupon the charge of etching gas in the first expansion chamber flows to the main chamber and the pressure of etching gas inside the first expansion chamber decreases, and to connect the second expansion chamber to the source of etching gas to be charged with a suitable amount of etching gas to sustain a constant flow of etching gas to the main chamber when connected thereto, and once charged to be isolated therefrom; and (c) following step (b), controlling the means for connecting to connect the charged second expansion chamber to the main chamber, whereupon the charge of etching gas in the second expansion chamber flows to the main chamber and the pressure of etching gas inside the second expansion chamber decreases, and to connect the first expansion chamber to the source of etching gas to be charged with a suitable amount of etching gas to sustain a constant flow of etching gas to the main chamber when connected thereto, and once charged to be isolated therefrom.
 28. The system of claim 27, wherein the controller is further operative for repeating steps (b) and (c).
 29. The system of claim 28, wherein: step (c) includes the second expansion chamber being connected to supply etching gas to the main chamber before the pressure of the etching gas in the first chamber decreases below a level sufficient to sustain a constant flow of etching gas to the main chamber; and when an instance of step (b) follows an instance step (c), step (b) includes the first expansion chamber being connected to supply etching gas to the main chamber before the pressure of the etching gas in the second chamber decreases below a level sufficient to sustain a constant flow of etching gas to the main chamber.
 30. The system of claim 29, wherein: step (c) includes the first and second expansion chambers being both momentarily connected to the main chamber; and when an instance of step (b) follows an instance step (c), step (b) includes the first and second expansion chambers being both momentarily connected to the main chamber.
 31. The system of claim 30, further including means for preventing etching gas from flowing from the first expansion chamber to the second expansion chamber, and vice versa, when the first and second expansion chambers are both connected to the main chamber.
 32. The system of claim 31, wherein the means for preventing includes at least one of: a sensor for measuring a flow direction of etching gas between the first and second expansion chambers, said sensor coupled to the controller which is responsive thereto for controlling the means for connecting to prevent the flow etching gas from the first expansion chamber into the second expansion chamber and vice versa; or at least one check valve between the first and second expansion chambers.
 33. The system of claim 27, further including at least one of: means for controlling a rate of flow of gas into the main chamber; or means for controlling a pressure of gas in the main chamber.
 34. The system of claim 27, further including a source of mixing gas(es), wherein the controller is operative for controlling the means for connecting to selectively connect each expansion chamber to the source of mixing gas(es). 